<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        .box {
            width: 200px;
            height: 200px;
            position: relative;

        }

        .mask {
            width: 200px;
            height: 200px;
            top: 0;
            left: 0;
            position: absolute;

        }

        .box:hover .mask {
            background-color: rgba(0, 0, 0, .3);
            display: 1;
        }
    </style>
</head>

<body>
    <div class="box">
        <div class="mask"></div>

    </div>
</body>

</html>